The present invention relates to a light diffusion sheet and a method of producing the same as well as a screen using the light diffusion sheet.
In recent years, large-size screen display devices, such as liquid crystal displays, plasma displays, and front or rear projection displays, have attracted attention, and those for not only commercial use but also domestic use are spreading. Especially, the projection type display devices have drawn great attention since large-size screen display devices may be achieved at relatively low cost.
The projection type display device employs a method of image projection in which a ray of light emitted from a light source is modulated by a device, such as a transmissive liquid crystal panel, a liquid crystal on silicon (LCOS), a digital micromirror device (DMD), or a grating light valve (GLV), to form an image light, and the image light is passed through optics system, such as a reflector or a lens, and projected on a screen.
The screen for projector is roughly classified into a screen for front projector such that image light is projected from the front side of the screen to see the reflected light of the projected light by the screen, and a screen for rear projector such that image light is projected from the back side of the screen to see the transmitted light through the screen from the front side of the screen. The screens of any types are required to have excellent recognizability and a large viewing angle.
Therefore, in the screens of any types, generally, a light diffusion sheet for scattering light is formed on the surface of the screen, and the light diffusion sheet makes image light be uniformly diffused and emitted from the entire effective region of the screen.
As the light diffusion sheet, two types of diffusion sheets roughly classified, i.e., an isotropic diffusion sheet and an anisotropic diffusion sheet are generally known, and the anisotropic diffusion sheet has attracted attention because it is able to diffuse incident light in a predetermined amount only in the desired direction to improve the luminance. Particularly, when the diffusion sheet is used in a projector image screen, the visual field in the horizontal direction is more important than the visual field in the vertical direction, and therefore the development of an anisotropic diffusion sheet having strong diffusing power in the horizontal direction is in progress.
As a method of producing an anisotropic light diffusion sheet, there have in the past been a method in which a speckle pattern, which is caused when a roughened surface is irradiated with a coherent light flux, is formed in a photosensitive resin to produce a light diffusion sheet (see, for example, Japanese Patent Application Publication No. Sho 53-51755, and Japanese Patent Application Publication No. 2001-100621), a method in which a mask is prepared and burnt on a photosensitive resin to produce a light diffusion sheet, and a method in which the surface of a mold base material, such as a metal or a resin, is directly ground by mechanical processing to prepare a mold having a finely uneven surface formed, and the surface form of the mold is transferred to an ultraviolet curing resin or the like to produce a light diffusion sheet.
In addition, there have been a method in which a composition comprising resin particles dispersed in a resin binder is applied to a transparent substrate to produce a light diffusion sheet, and a method in which a mold having an uneven surface is prepared by subjecting a mold base material to sandblasting, and the surface form of the mold is transferred to an ultraviolet curing resin or the like to produce a light diffusion sheet.
By the method in which the form of a mold is transferred to a sheet, a light diffusion sheet having specific diffusion properties may be mass-produced at a low cost; however, the diffusion performance required for the screen varies depending on the arrangement or ambient light intensity in the space in which the display device is placed and hence light diffusion sheets having various diffusion performance are needed, and therefore manufacturing apparatuses exclusive to the respective diffusion sheets having various properties and the corresponding molds must be prepared.
The preparation of the molds exclusive to the respective diffusion sheets has shortcomings per mold production method. For example, in the method in which a speckle interference or mask pattern is burnt on a photosensitive resin to produce a light diffusion sheet, when a plurality of light diffusion sheets are produced, a mold for duplicating light diffusion sheets from the photosensitive resin must be prepared, and the exposure of the photosensitive resin requires an exposure time as long as several hours to several days per m2. Further, after the exposure, a step for duplicating light diffusion sheets using the photosensitive resin, a step for imparting electrical conduction, a step for electroforming, and the like are needed, and thus a considerably prolonged time and high cost are required in the production of the mold for light diffusion sheet.
In the method in which a light diffusion sheet is produced using a mold prepared by mechanically grinding the surface of a mold base material, there are issues in that the precision of grinding is not satisfactory, the tools are damaged during the grinding, a very long time is required for the grinding, and the facility for the grinding is large.
On the other hand, in the method in which a light diffusion sheet is produced using a mold prepared by sandblasting, a long time or high cost is not required for the production of the mold, but the issue in that the apparatuses for producing the respective molds having various properties are needed is not solved.
In the light diffusion sheet produced by transferring the form of a mold to a sheet, a defect may be caused in part of the uneven form of the surface, and, in this case, reflection or transmission of light occurs at the defect site without diffusion. When a screen is formed using such a light diffusion sheet, strong reflection or transmission occurs only at the defect site to cause the light to glare, leading to an issue in which the glaring light is dangerous when using a high power light source, such as a laser.